read_ximg: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/lib/mrtrix/bin/read_ximg - from package: mrtrix

You may install the required package with command apt-get install {package}

Error in other languages:
read_ximg: bevel nie gevind nie
read_ximg: comandă negăsită
read_ximg: comando não encontrado
read_ximg: commande introuvable
read_ximg: command not found
read_ximg: käsku ei ole
read_ximg: không tìm thấy lệnh
read_ximg: komanda nerasta
read_ximg: Kommando nicht gefunden.
read_ximg: kommandot finns inte
read_ximg: komut yok
read_ximg: nie znaleziono polecenia
read_ximg: níor aimsíodh an t-ordú
read_ximg: no se encontró la orden
read_ximg: no s'ha trobat l'ordre
read_ximg: opdracht niet gevonden
read_ximg: parancs nem található
read_ximg: perintah tidak ditemukan
read_ximg: príkaz nenájdený
read_ximg: příkaz nenalezen
read_ximg: команда не найдена
read_ximg: командата не е открита
read_ximg: 命令找不到
read_ximg: コマンドが見つかりません